The present invention relates generally to a catalytic process and particularly to the treatment of perfluoroalkanes. Perfluoroalkanes represent a specific group of halogen-containing compounds consisting of straight, branched and cyclic alkanes that are composed of only carbon and fluorine atoms.
Perfluoroalkanes refer to a specific group of halogen-containing compounds that are composed of only carbon and fluorine atoms and do not possess double or triple bonds. Perfluoroalkanes differ from, for example, chlorofluorocarbons (CFC""s), hydrochlorofluorocarbons (HCFC""s) and hydrofluorocarbons (HFC""s) in that perfluoroalkanes do not contain hydrogen, chlorine or heteroatoms other than fluorine. Perfluoroalkanes are released to the environment during certain industrial processes, such as electrolytic aluminum smelting for example, as by-products during the manufacture of tetrafluoroethylene, and during semiconductor manufacturing processes. Examples of perfluoroalkanes include carbon tetrafluoride (CF4), hexafluoroethane (C2F6), octafluoropropane (C3F8), octafluorocyclobutane (C4F8) and decafluoroisobutane (C4F10) Perfluoroalkanes represent some of the most stable compounds known (Kiplinger et al. Chem. Rev. p. 373 (1994). The stability of perfluoroalkanes makes these compounds difficult to decompose or convert to useful products, such as for example the conversion of perfluoroalkanes to perfluoroalkenes. Also, this highly stable characteristic make perfluoroalkanes released into the atmosphere undesirable because of their contribution to global warming effects.
A number of catalysts and catalytic processes have been reported for the decomposition of halogen-containing organic compounds. A review of the literature reveals that the majority of these catalysts and catalytic processes focus on the decomposition of chlorine-containing compounds, or the destruction of organic compounds which contain only chlorine and fluorine. Bond and Sadeghi, in an article entitled xe2x80x9cCatalyzed Destruction of Chlorinated Hydrocarbonsxe2x80x9d, J.Appl. Chem. Biotechnol, p. 241 (1975), report the destruction of chlorinated hydrocarbons over a platinum catalyst supported on high surface area alumina.
Karmaker and Green, in an article entitled xe2x80x9cAn investigation of CFC12 (Ccl2F2) decomposition on TiO2 Catalyst,xe2x80x9d J. Catal, p. 394 (1995), report the use of a TiO2 catalyst to destroy CFC12 at reaction temperatures between 200 and 400xc2x0 C. in streams of humid air.
Bickel et al, in an article entitled xe2x80x9cCatalytic Destruction of Chlorofluorocarbons and Toxic Chlorinated Hydrocarbonsxe2x80x9d, Appl. Catal B:Env. p. 141 (1994), report the use of a platinum catalyst supported on phosphate-doped zirconium oxide for the destruction of CFC113 (Cl2FCCClF2) in air streams. The catalyst was able to achieve greater than 95% destruction of CFC113 at reaction temperature of 500xc2x0 C. for approximately 300 hours of continuous operation.
Fan and Yates, in an article entitled xe2x80x9cInfrared Study of the Oxidation of Hexafluoropropene on TiO2,xe2x80x9d J. Phys. Chem., p. 1061 (1994), report the destruction of a perfluoroalkene over TiO2. Perfluoroalkenes differ from perfluoroalkanes in that they contain a carbon-carbon double bond. Although the catalyst was able to readily destroy hexafluoropropylene (C3F6), the loss of titanium, as TiF4, was evident. The formation of TiF4 would undoubtedly lead to deactivation of the catalyst.
Farris et al, in an article entitled xe2x80x9cDeactivation of a Pt/Al2O3 Catalyst During the Oxidation of Hexafluoropropylene,xe2x80x9d Catal. Today, p. 501 (1992), report the destruction of hexafluoropropylene over a platinum catalyst supported on a high surface area alumina carrier. Although the catalyst could readily destroy hexafluoropropylene at reaction temperatures between 300 and 400xc2x0 C., deactivation of the catalyst, resulting from the transformation of aluminum oxide to aluminum trifluoride, was severe.
Campbell and Rossin, in a paper entitled xe2x80x9cCatalytic Oxidation of Perfluorocyclobutene over a Pt/TiO2 Catalyst,xe2x80x9d presented at the 14th N. Am. Catal. Soc. Meeting (1995), reported the use of a platinum catalyst supported on high surface area TiO2 carrier to destroy perfluorocyclobutene (C4F6) at reaction temperatures between 320 and 410xc2x0 C. The authors note than even at a reaction temperature of 550xc2x0 C., no conversion of perfluorocyclobutane (C4F8), a perfluoroalkane, could be achieved using the Pt/TiO2 catalyst. Results presented in this study demonstrate that perfluoroalkanes are significantly more difficult to transform than perfluoroalkenes.
Nagata et al, in a paper entitled xe2x80x9cCatalytic Oxidative Decomposition of Chlorofluorocarbons (CFC""s) in the Presence of Hydrocarbonsxe2x80x9d, Appl. Catal. B:Env., p. 23 (1994), report the destruction of 1,1,2-trichloro 1,2,2-trifluoroethane (CFC113), 1,2 dichloro 1,1,2,2-tetrafluoroethane (CFC114) and chloropentafluoroethane (CFC115) in the presence of hydrocarbons using a xcex3-alumina catalyst impregnated with vanadium, molybdenum, tungsten and platinum. The decomposition of the CFC""s became more difficult as the number of carbon atoms in the CFC molecule decreased. However, results indicate that as the number of chlorine atoms in the molecule are decreased by replacement with fluorine, the compounds become increasingly more difficult to decompose.
Burdeniue and Crabtree, in an article entitled xe2x80x9cMineralization of Chlorofluorocarbons and Aromatization of Saturated Fluorocarbons by a Convenient Thermal Processxe2x80x9d, Science, p. 340 (1996), report the transformation of cyclic perfluoroalkanes to perfluoroarenes via contact with sodium oxalate to yield sodium fluoride as a reaction product. Both reactions, however, are slow and non-catalytic, since sodium oxalate is stoichiometrically consumed (via transformation into NaF) during the course of the reaction.
This process would not be able to destroy perfluoroalkanes present in streams of air, since the oxygen and/or moisture in the air would readily convert the sodium oxalate to sodium oxide.
The present invention is directed to processes for the transformation of perfluoroalkanes and for catalytic compositions used therein. More particularly, the present invention is directed to a process for the transformation of perfluoroalkanes comprising contacting the perfluoroalkanes with aluminum oxide. According to one embodiment, the perfluoroalkane is contacted with aluminum oxide at a temperature ranging from about 400xc2x0 C. to about 1000xc2x0 C. According to a further embodiment of the invention, the process for the transformation of a perfluoroalkane comprises contacting the perfluoroalkane with aluminum oxide at a temperature ranging from about 550xc2x0 C. to about 800xc2x0 C.
The present invention is also directed to a process for the transformation of perfluoroalkanes comprising contacting the perfluoroalkane with aluminum oxide wherein said aluminum oxide is stabilized, for example, with an element selected from the group consisting of barium, calcium, cerium, chromium, cobalt, iron, lanthanum, phosphorus, magnesium, nickel, silicon, titanium, yttrium, and zirconium. In a further embodiment, the aluminum oxide may be stabilized with molybdenum; tungsten, and vanadium.
According to another embodiment of the present invention, the process of transforming a perfluoroalkane comprises contacting the perfluoroalkane with aluminum oxide in the presence of water and an oxidizing agent.
According to a further embodiment of the invention, the process comprises contacting the perfluoroalkane with a composition comprising aluminum oxide, cobalt, for example, less than 50% by weight cobalt, and, for example, less than 50% by weight of at least one element selected from the group consisting of cerium, titanium, and zirconium.
According to another embodiment of the present invention, the invention is directed to a composition for the transformation of perfluoroalkanes comprising aluminum oxide, and at least one element selected from the group consisting of barium, calcium, cerium, chromium, cobalt, iron, lanthanum, magnesium, molybdenum, nickel, tin, titanium, tungsten, vanadium, yttrium, and zirconium.
According to another embodiment of the present invention, the invention is directed to a composition for the transformation of a perfluoroalkane comprising aluminum oxide, cobalt, for example, less than 50% by weight, and at least one element selected from the group consisting of cerium, titanium, and zirconium, for example, less than 50% by weight of one of said elements.
According to a still further embodiment of the present invention, the invention is directed to a composition for the transformation of a perfluoroalkane comprising aluminum oxide. In one embodiment of the present invention, the aluminum oxide may be stabilized with, for example, an element selected from the group consisting of barium, calcium, cerium, chromium, cobalt, iron, lanthanum, phosphorus, magnesium, nickel, silicon, titanium, yttrium, and zirconium. According to a still further embodiment, the aluminum oxide may be stabilized with an element selected from the group consisting of molybdenum, tungsten, and vanadium.
The present invention relates generally to a novel catalytic process for the transformation of perfluoroalkanes, such as for example, those vented to the atmosphere during chemical process operations. Examples of these processes include perfluoroalkanes generated during electrolytic aluminum smelting, tetrafluoroethylene manufacture, and during semiconductor manufacture. The process according to the present invention employs aluminum oxide as a catalyst, where the aluminum oxide may be of several phases, such as for example gamma, alpha, delta, kappa and theta, or a combination of phases, with the gamma phase being the preferred phase of aluminum oxide. While testing has shown aluminum oxide will readily destroy perfluoroalkanes at reaction temperatures between 400 and 1,000xc2x0 C., the useful life-time of the catalyst appears to be limited due to deactivation resulting from an interaction between fluorine atoms liberated during the destruction of the perfluoroalkane and elemental aluminum which comprises the catalyst.
A preferred catalyst composition comprises aluminum oxide with the addition of between 0.01 and 50% of one or more elements selected from a group which include barium, calcium, cerium, chromium, cobalt, iron, lanthanum, phosphorus, magnesium, nickel, silicon, titanium, yttrium and zirconium. A more preferred catalyst consists of aluminum oxide containing cerium, titanium or zirconium, and cobalt. Other useful components which may be added to the aluminum oxide include molybdenum, tungsten or vanadium.
The catalyst may be used in any configuration or size which sufficiently exposes the catalyst to the gas stream being treated. The catalyst composition may be configured in many typical and well-known forms, such as for example, pellets, granules, rings, spheres or cylinders. Alternatively, the catalyst composition may take the form of a coating on an inert carrier, such as ceramic foams, spheres or monoliths. The monolithic form may be preferred when it is desired to reduce the pressure drop through the system or minimize attrition or dusting.
The additional components may be dispersed onto the aluminum oxide by contacting the aluminum oxide with an aqueous or non-aqueous solution containing one or more of these components. Once the impregnation step is completed, the resulting material may be dried and/or calcined. If two or more additional components are to be employed, a preferred method of catalyst preparation may involve sequentially impregnating the aluminum oxide with a solution containing one or more of these added components followed by drying and/or calcining the resulting material. Once this step is completed, the resulting material may be impregnated with a solution containing the same or other of these additional components, followed again by drying and calcining the resulting material. These steps may be repeated until all the additional components have been added in the amount desired. In all cases, the solution containing the additional components may be aqueous or non-aqueous.
Alternatively, the additional components may be added during the preparation of the aluminum oxide. In this instance, the catalyst is prepared by slurrying pseudoboehmite aluminum oxide (Al2O3.1.5H2O) in an aqueous or non-aqueous liquid with an appropriate mixing device and adjusting the pH to between 1.0 and 6.0 using an appropriate acid, such as nitric, formic or acetic. Once mixed, one or more additional components may be added to the slurry. These additional components may be added as solid metal salts, such as nitrates, acetates, oxalates, chlorides, halides, etc., or may be added as small metal or metal oxide particles, such as for example cerium oxide. Once mixed, the slurry may be aged, if desired, or used directly in the manufacture of beads, particles, spheres, rings, etc., or used to coat an inert ceramic substrate, such as a monolith. Following manufacture or coating of the inert ceramic substrate, the resulting material must be calcined at a temperature between 350 and 900xc2x0 C., with the preferred calcination temperature being between about 500 and 600xc2x0 C.
It should be noted that the additional elements added to the aluminum oxide should be highly dispersed throughout the particular configuration used.
If one wishes to manufacture catalyst particles, for example, the resulting slurry described above is first dried, then calcined at a temperature sufficient to form the desired aluminum oxide phase, such as between 500 and 600xc2x0 C. if one wishes to form the gamma phase of aluminum oxide. Once calcined, the resulting material may be crushed and sieved to the desired mesh size range.
Alternatively, if the monolithic form of the catalyst is desired, the monolithic form may be prepared, for example, by dipping the monolithic substrate into a pseudoboehmite slurry, or a pseudoboehmite slurry containing one or more additional components. Excess slurry may be removed from the channels of the monolithic substrate using an air knife according to procedures well known to one skilled in the art. The catalyst-coated monolith is then dried and calcined at a temperature suitable to achieve the desired form of aluminum oxide. The wash coating procedure can be repeated as often as required until the desired loading of catalyst is achieved. It is desirable that the amount of the catalyst composition coated onto the monolith be in the range of about 25 to about 350 g/liter.
The novel catalytic process of the present invention preferably involves passing a gas stream containing one or more perfluoroalkanes, an oxidizing agent, such as air, and water vapor through a catalyst bed containing a catalyst composition as described herein and heated to the desired operating temperature. The flow rates through the system should be sufficient to allow for greater than at least 80% and preferably greater than 90% so destruction of the perfluoroalkane(s) present in the stream. Thus, the gas hourly spaced velocity (GHSV) can vary significantly over the range of about 500 to about 300,000 hxe2x88x921, and preferably in the range of about 1,000 to about 20,000 hxe2x88x921. The process described herein may be operated at temperatures between about 400xc2x0 C. to about 1,000xc2x0 C., with the preferred temperature range between about 500 and 800xc2x0 C.
The process described according to the present invention is also applicable to the injection of gaseous or liquid phase perfluoroalkanes or mixtures of perfluoroalkanes into a gas stream, including an oxidizing agent, such as air for example, and water. The gas stream temperature and flow rate, and rate of perfluoroalkane(s) injection, may be controlled to achieve the desired concentration of perfluoroalkane(s) to be treated. The resulting gas stream containing the perflouoroalkane(s) is then contacted with the catalyst compositions described herein.
It should also be noted that after the gas stream has been treated in accordance with the present invention, further treatment, if desired, may be necessary to remove hydrofluoric acid (formed during the decomposition of the perfluoroalkanes in the presence of an oxidizing agent and water) from the effluent stream. If the concentration of hydrofluoric acid in the effluent stream is deemed unacceptable, conventional collection or abatement processes, such as caustic scrubbing, may be employed to avoid venting acid gases directly into the atmosphere.
In the more preferred embodiments of the present invention, a relatively small percentage, such as about 0.01 to 5% of a base or noble metal, appear to aid the complete conversion of carbon monoxide to carbon dioxide in the reaction products. In this connection it has not been observed that noble metals perform better than base metals.
Under certain operating conditions, aluminum oxide alone may not possess the required useful life and may degrade faster than desirable warranting replacement of the catalyst following a short period of operation. However, the aluminum oxide may be used to treat process streams containing low to moderate concentrations of perfluoroalkanes using a fluidized bed reactor configuration employing aluminum oxide particles of a fluidizable size. Using this reactor configuration would allow for removing catalyst from the reactor during process operation while simultaneously adding fresh catalyst in order to maintain a satisfactory threshold activity of the reactor over a sustained useful period.
The compositions of the catalysts recited herein are stated in percent by weight unless otherwise indicated and were calculated based upon the elements described. When the metal component or components were added by wet impregnation techniques, the weight percent of the metal component(s) were calculated from the concentration of metal(s) within the impregnation solution and the amount of impregnation solution used to prepare the catalyst. When the metal component or components were added to the aluminum oxide precursor (e.g. pseudoboehmite) slurried in water, the weight percent of the metal component(s) were calculated from the amount of aluminum oxide precursor and the amount of metal(s) present within the slurry, and the weight loss upon ignition of the aluminum oxide precursor (e.g. 20-30% for pseudoboehmite).
The concentration of CO, CO2 and perfluoroalkane in the reactor effluent in the following examples described herein were determined using gas chromatographic techniques employing packed columns and both thermal conductivity and flame ionization detectors. The above analytical techniques are well known to those skilled in the art.
The additional components added to the aluminum oxide catalyst appear to improve the effective useful life of the aluminum oxide catalyst by maintaining the reactivity of the aluminum oxide at a high level for greater periods of time. This may be referred to as a stabilizing effect.
In view of the above description and the examples of the process according to the present invention which follow, it should be understood by those skilled in the art that the present invention provides processes and catalyst compositions which very effectively transform perfluoroalkanes.